PRIVATE stylefile:c:!temp!~pr000006a80001001d2e9c000.STY The countdown for NASA’s Space Shuttle Discovery mission was set to officially begin June 28 following a vigorous debate over the whether the orbiter’s planned July 1 launch should be postponed due to a nagging safety concern.

Discovery was cleared June 17 for launch to the international space station following a two-day flight readiness review during which NASA’s chief engineer and top safety official raised red flags over a potential foam-shedding issue with the shuttle’s external tank. Insulation from the tank can shake loose during liftoff, posing a threat to the heat shielding that protects the orbiter during re-entry.

NASA’s associate administrator for safety and mission assurance, Bryan O’Connor, and NASA Chief Engineer Christopher Scolese both voted “no-go” for the launch on the grounds that flying the external tank without modifications could result in crippling damage to the orbiter. But both said they agreed with NASA Administrator Mike Griffin that the mission does not pose an unacceptable risk to the astronauts because even under the worse-case scenario they would still make it to the space station, where they could either repair the orbiter or await rescue by another shuttle or multiple Russian Soyuz capsules.

Discovery’s STS-121 mission is the first shuttle launch in nearly a year and only the second since the Space Shuttle Columbia broke up during re-entry in February 2003. Columbia’s seven astronauts were unaware that the large chunk of foam insulation that broke away on liftoff two weeks earlier had fatally breached the orbiter’s heat shield.

NASA responded to the disaster by hardening the orbiter, redesigning the external tank to reduce foam shedding, and installing new ground-based and on-orbit inspection capabilities to detect damage to the shuttle. NASA also has worked on techniques and materials for repairing a damaged shuttle in space but does not have any certified capabilities in hand. Despite these efforts, large foam pieces were seen falling from Discovery’s external tank during its liftoff last summer, marring what NASA otherwise considered a good return to flight and prompting the agency to once again ground the fleet for more modifications.

NASA decided late last year to eliminate the part of the tank that had been the source of Discovery’s trouble, a foam-insulated windbreak, known as a protuberance air load ramp, that was added to the tank years ago to protect pressurization lines and a long cable tray from the aerodynamic stresses of flight. Wind tunnel tests conducted earlier this year validated the change.

But another tank modification NASA was considering — redesigning the 37 foam-insulated brackets, or ice-frost ramps, that hold the pressure and power lines in place — did not fare as well. With the agency’s engineers divided on how to proceed, Griffin weighed in and broke the impasse. In April, flanked by his two top shuttle officials, Griffin announced that NASA would launch this summer with the ice-frost ramps left the way they are, but said the agency would continue to look for a better design that could be implemented within several flights.

Ice- frost ramps remained a hot topic among NASA engineers heading into the STS-121 flight readiness review and were put on the agenda for the first day of the meeting. O’Connor and Scolese argued for postponing the mission until the ramps could be redesigned. Their case was bolstered by the agency’s engineering directorate classifying the risk associated with the ramps as “probable/catastrophic,” meaning the issue is deemed likely to result in the loss of an orbiter sometime over the course of 100 flights . NASA plans to fly no more than 18 shuttle missions, including STS-121, before retiring the fleet in 2010.

Griffin told reporters after the two-day meeting that he did not agree that the risk passes the “probable/catastrophic” threshold and that even in a worst-case scenario astronauts would not be in any immediate danger. “If we are unlucky and we have a debris event on ascent, it will not impede the ascent; the crew will arrive safely on orbit and then we will begin to look at our options, whether those include repair, launch on need, extended safe haven on the station, asking our Russian partners for help, [or] maybe some or all of the above,” Griffin said.

The space station is equipped to accommodate Discovery’s crew for 80-82 days and perhaps longer if fresh supplies are sent up on Russian Progress ships and some of the crew members are brought home early on Russian Soyuz vehicles.

Griffin said any additional risk NASA is taking by pressing ahead with the flight is both minimal and necessary in order to finish the space station before retiring the shuttle.

“This president’s budget will not carry funding for shuttle vehicles beyond 2010. So if we’re going to fly, we need to accept some programmatic risk and get on with it. Again, I will point out, for me to accept some programmatic risk to do this is not the same as accepting crew risk, which we believe we are not doing.”

O’Connor and Scolese told reporters June 21 they agreed with Griffin’s reasoning and would not appeal the decision to launch. The two also suggested their objections were intended to raise the issue to Griffin’s level so that he could make the final decision about whether to proceed with a mission that in a worst-case scenario likely would result in the end of the shuttle program.

“I believed this should be elevated to the administrator, and I felt very confident that he knew the risks with his eyes open,” O’Connor said. “If the administrator could accept it, I felt I was not going to throw my badge down. We now go forward to get this vehicle off the launch pad.”